1. Field of the Invention
The present invention relates to a surface acoustic wave filter, especially to a surface acoustic wave filter to be used in a high frequency region.
2. Related art of the Invention
In recent years, the researches of the surface acoustic wave elements have been briskly effected so as to use for filters. The surface acoustic wave elements, especially, the surface acoustic wave filters are positively developed especially by the development and higher frequency of the recent portable communication unit.
Several types of methods of composing a filter with surface acoustic wave elements have been known in high frequency bands, especially in several hundred Mhz. As representative types, there are such a ladder type of making filters with the use of a plurality of surface acoustic wave resonators as shown in Japanese Laid-Open Patent Application Tokkaisho No. 52-19044, such IIDT type as shown in Japanese Laid-Open Patent Application Tokkaisho No. 58-154917, such inline resonator coupled filter as shown in Japanese Laid-Open Patent Application Tokkaihei No. 3-222512, which are set in adjacent relation to use the coupling among the resonators.
In recent years, a smaller type of surface acoustic wave filters superior in function are demanded for a smaller potable communication unit apparatus. In the major using locations, they are often used as interstate filters for transmission or reception, or output filters for local oscillators. Recently the ladder type among them is noteworthy in characteristics. The ladder type filter is constituted as FIG. 17, multi-stages of which are usually used. Reference numeral 2001 is an input terminal. Reference terminal 2002 is an output terminal. Reference numeral 2003 is a surface acoustic wave resonator which is in series with the signal line 100. Reference numeral 2004 is a surface acoustic wave resonator which is located between the signal line 100 and the earth. The filter characteristics are constructed by controlling the resonance frequency and anti-resonance frequency of the series resonator and the parallel resonator as shown in FIG. 18. The "A" point which is a lower attenuation pole out the passband of the filter which is the resonance frequency of the parallel resonator, while the "B" point which is a higher attenuation pole out of the passband corresponds to the anti-resonance frequency of the series resonator. Also, the passband is determined by the anti-resonance frequency of the parallel resonator and the resonance frequency of the series resonator.
As performance to be desired for the recent filters, lower loss is required within the passband and higher attenuation is required in the stopband.
But two demands are contrary to each other. When the low loss is pursued, the attenuation becomes smaller. When the attenuation is made larger, the loss is likely to increase. In the characteristics of the filter, the loss within the band and the attenuation out of the band are often regulated across some frequency ranges. But in the case of the surface acoustic wave filter, the attenuation out of the band is deteriorated when it was away from the passband as in the C point of FIG. 18. It was often difficult to make a low loss filter especially when the higher attenuation was required in a plurality of frequency bands. Intervals of the resonance frequency, anti-resonance frequency of the surface acoustic wave resonator are almost determined by the piezo-electric substrate to be formed by the surface acoustic wave resonator. Therefore, the band width as the filter is not free in degree as much as that. When a LiTaO.sub.3 substrate of 36.degree. Y cut X propagation generally used is used, 33 MHz was almost a limit in a filter with a 900 MHz band as a central frequency. When the band or more is required, the filter cannot be constructed as a defect.